X-ray images have traditionally been recorded by placing an X-ray absorbing scintillator layer and a photographic film adjacent the object, and illuminating the object with X-rays. X-rays that pass through the object cause the scintillator layer to emit light that selectively exposes the photographic film to form an image of the object.
Electronic X-ray imaging has been introduced relatively recently. In electronic X-ray imaging, either the photographic film is replaced by an array of electronic light detectors, or the scintillator layer and the photographic film are replaced by an array of electronic X-ray detectors. Electronic imaging offers a number of advantages of traditional photographic imaging. One important advantage is that the image can be viewed immediately, without the need to wait for the photographic film to be developed. Additionally, electronic X-ray images can be stored, transmitted, and analyzed electronically. Still picture compression techniques can be used to reduced the amount of storage space or transmission time required.
Crystalline silicon photon detectors can be used as electronic X-ray detectors. However, X-ray detectors having a layer of scintillator material coupled to a p-i-n light detector are lower in cost, higher in efficiency, and have greater radiation hardness. In the p-i-n light detector in such X-ray detectors, a P-type layer, an N-type layer, and an intrinsic region between the P-type layer and the N-type layer are formed in a layer of hydrogenated amorphous silicon. At present, a two-dimensional amorphous silicon X-ray detector array covering the size of a conventional X-ray film (up to about 360 mm by 430 mm) is not feasible from the point of view of cost. To overcome this difficulty, the object is mechanically scanned using a linear (one-dimensional) electronic X-ray detector.
To scan the object, either the linear X-ray detector is moved linearly relative to the object, and the X-ray source is rotated to track the X-ray detector, or the X-ray source and the linear X-ray detector array are maintained in a fixed relationship to one another, and the object is moved linearly between them. With either scanning technique, the electrical outputs of all the X-ray detecting elements ("pixels") of the linear X-ray detector provide one line of a rasterized X-ray image. The electrical outputs of all the pixels of the linear X-ray detector for each line of the rasterized X-ray image are stored. When the scan is completed, the rasterized X-ray image is derived from the stored electrical outputs.
Linear amorphous silicon light detectors have recently become more efficient and lower in cost due to their widespread use in facsimile machines. This application has also resulted in the development of techniques for integrating the linear light detector with its associated readout electronics. A linear X-ray detector can be made using a layer of a suitable scintillator, such as a layer of thallium-activated cesium iodide, and a linear amorphous silicon light detector. However, the spatial resolution of such an arrangement is usually less than the spatial resolution of the light detector because the light generated in the scintillator diffuses laterally in the scintillator layer before it reaches the light detector.
In U.S. Pat. No. 5,171,996, the disclosure of which is incorporated herein by reference, one of the inventors (Perez-Mendez) describes a method of making an X-ray detector in which the scintillator layer is grown on the surface of the light detector in columns perpendicular to the surface. Each column of the scintillator layer acts as a light guide, which significantly reduces the lateral diffusion of the light generated in the scintillator layer. This increases the spatial resolution of the X-ray detector to almost that of the light detector.
Additional information about an object can be determined by making X-ray images of the object at a number of different X-ray energies. Electronic X-ray detection is particularly advantageous for making images of an object at different X-ray energies because it allows additional information to be determined by performing arithmetic operations on the electrical outputs of the pixels obtained at the different X-ray energies. This can be done especially conveniently if the pixel outputs are digitized before processing. However, current techniques of using linear X-ray detectors require that a separate scan be performed at each X-ray energy to gather a set of pixel outputs at each X-ray energy. This means that more time is required to generate a multiple-energy X-ray image than is required to generate a single-energy X-ray image. In medical applications, the need for a scan at each X-ray energy increases the X-ray exposure of the patient. Moreover, additional scans may be needed if the patient moves between successive scans. In all applications, the need to make multiple spatially coherent scans demands increased accuracy from the scanning system and increases the potential X-ray exposure of personnel. Also, multiple scans cannot be used to produce multiple-energy images of moving objects, such as the heart.
The disadvantages of current techniques for generating multiple-energy X-ray images could be overcome by using an electronic X-ray detector that does not require an independent scan to generate an X-ray image at each X-ray energy.